This invention relates to averaging circuits for processing signals which are periodically and intermittently input, and more particularly to an averaging circuit for an optical intensity signal received by an optical wave range finder.
One example of the optical system of an optical wave range finder is as shown in FIG. 1. A modulated light beam is emitted from a light emitting diode 2 in the optical wave range finder 10. The modulated light beam is applied through a prism 9 and an objective lens 8 to a reflector 1 such as a corner cube placed at a measurement point. The modulated light beam 5 reflected by the reflector 1 is received by a light receiving element 3, and the phase difference between the emitted and received modulated light beams is measured and converted into a distance.
In this case, in order to eliminate a phase measurement error which is caused in the signal receiving circuit provided in the rear stage of the light receiving element, a method is employed in which a calibration light beam 4 (hereinafter referred to as "the internal light beam") is provided in the optical wave range finder 10, and the intensities of the internal light beam 4 and the modulated light beam 5 reflected from the reflector 1 (hereinafter referred to as "the external light beam") are controlled by arranging a light quantity adjuster 6 such as a variable density filter or a diaphragm in one or two optical paths as shown in FIG. 1, so that the external light beam 5 and the internal light beam 4 as received are made equal in intensity.
The internal light beam and the external light beam 5 are alternately produced by means of a light chopper 7 or the like. Therefore, the production sequence of the internal light beam 4 and the external light beam 5 is as shown in FIGS. 2a and 2b.
FIG. 2a shows a pulsive external light beam, the light beam being provided at times indicated by "H". FIG. 2b shows the pulsive internal light beam, this light beam also being provided at the times indicated at "H" in FIG. 2b. The external light beam 5 and the internal light beam 4 alternately provided in this fashion are received by the light receiving element 3. The intensities of the light beams 4 and 5 thus received are detected (the detection signals being referred to as "light quantity signals" hereinafter), and the intensities thus detected are made equal by means of the light quantity adjuster 6. This control is carried out manually or automatically. In order to control the light quantity adjuster 6 with respect to the production sequence of the external and internal light beams 5 and 4, in general, hold circuits for the light quantity signals of the external light beam 5 and the internal light beam 4 are disposed, so that, during reception of the external light beam 5, the light quantity signal of the external light beam 5 being received is compared with the light quantity signal of the internal light 4 which was held at the time of the internal light beam 4. Conversely, at the time the internal light beam is received, the internal light quantity signal being received is compared with the external light quantity signal previously held. A comparison output signal representative of the difference between the light quantity signals or their coincidence is used to control the light quantity adjuster 6. When coincidence is obtained, the light quantity adjuster 6 is stopped. In the case of manual control, the comparison output signal is displayed to inform the operator of the aforementioned difference or coincidence. In the case of automatic control, feedback control is effected according to the comparison output signal. In such a case, the internal light beam 4 is not affected by atmospheric conditions because it is inside the optical wave range finder, but the external light beam 5 is affected thereby (cf. FIG. 2d ). Especially in the case when there is schlieren, the intensity of the external light beam received is greatly varied, as a result of which the external light quantity signal varies greatly as indicated by the solid line in FIG. 2c. Accordingly, the comparison output signal is affected so greatly that the display or control is adversely affected.
In order to prevent the comparison output signal from being affected as described, an averaging circuit such as an integration circuit or a time constant circuit is used, so the external light beam signal in one period is averaged as indicated by the dotted light in FIG. 2c, or the range of coincidence is increased, i.e., the control gain is decreased. However, the former method is not effective against variations over many periods, and the latter method is low in measurement accuracy, being poor in coincidence accuracy.